Lattice – or commonly called deformation cage outside of Blender.
A lattice consists of a three-dimensional non-renderable grid of vertices.
Its main use is to apply a deformation to the object it controls with a Lattice Modifier.
If the object is parented with Lattice Deform a Lattice Modifier is automatically applied.

The lattice should be scaled and moved to fit around your object in Object Mode.
Any scaling applied to the object in Edit Mode will result in the object deforming.
This includes applying its scale with Ctrl-A as this will achieve the same result as
scaling the lattice in Edit Mode, and therefore the object.